This invention relates to a method and apparatus for recording an image on a recording plate.
Lithographic printing plates for which this invention is particularly well suited typically comprise a rectangular aluminum substrate onto which is deposited a thin layer of photographic polymer. In addition a further thin protective coating of a polymer such as PVA is often added.
Recording of the image information onto such a plate is typically achieved in an image scanner using a scanned laser beam which hardens the photographic polymer in a selective manner. Following recordal of the image in the image scanner, the lithographic plate is later used within lithographic printing apparatus. A similar system for the recording of an image may also be used in connection with photographic plates and for the purposes of this invention both lithographic and photographic plates are described as a recording plate or plates.
In order to achieve high productivity it is desirable to use an image scanner in conjunction with automatic feeding apparatus to sequentially feed recording plates from a store and deliver them to the image scanner via a transport system. Typically the store contains a stack of such recording plates separated by interleaved sheets which protect the sensitive surface of the plates on which the image will be recorded. The delivery of individual plates to the transport system is achieved by removing the top plate from the stack using a feed system. The stack of plates has a typical mass of up to 60 kg, all plates within the cassette being of equal size. It is desirable that the apparatus can supply these plates to the image scanner without manual intervention. Manual intervention is undesirable as it slows the overall process considerably and further increases the risk of exposure of the image recording surfaces to radiation. For similar reasons it is therefore desirable that the interleaved sheets are automatically removed from the plate feeding path and are placed in a receptacle for later disposal. These interleaved sheets are often made from paper which is considerably different in mechanical properties when compared with the recording plates.
The established practice in existing image processing systems is to withdraw the recording plates sequentially from the stack using vacuum-activated suckers. The suckers are mounted upon a movable gantry and engage the topmost recording plate, then lifting it clear of the stack. The gantry then moves the recording plate to a receiving position for feeding into the image scanner. Separating means are often provided for the removal of an interleaved sheet when this is the topmost constituent in the stack. For example such means may include a roller which is moved to engage with the topmost sheet and withdraw it from the stack towards a bin. However, due to adhesion between the topmost recording plate and one or more plates or interleaved sheets beneath, a multiple feed operation can occur which is required to be remedied by separation of the plates and sheets. Known methods of performing this separation include the blowing of air between the interfaces of the plates and sheets, or simply holding the topmost plate in an elevated position above the stack such that the action of gravity eventually causes separation of the mis-fed items beneath.
Typical problems with such a system are that oxygen depletion under the suckers causes degradation of the emulsion, and secondary vacuum effects lead to multiple plate feeding.
One major problem with the use of vacuum suckers is that they engage in a localized manner with the fragile coated surface of the recording plate. As the vacuum is supplied, the plate deforms locally in the areas under the suckers producing characteristic circular marks on the processed plate. A second problem that occurs in such vacuum systems, is that the vacuum pressure required to lift a plate vertically is different from that to lift a sheet. If the pressure is not adjusted between the feeds then, since the paper is porous, the suckers will lift an interleaved sheet as well as the plate below. The vacuum system must therefore be capable of distinguishing between plates and sheets and, having set the correct vacuum level and lifted the topmost item, must confirm that the separation has been achieved before the selected plate or sheet is moved away from the stack. The time required to perform these steps is relatively large and in many cases rate limiting to the productivity of the complete system. This method is also unable to be optimized for a full range of plate sizes and thicknesses. In addition the apparatus is costly and often unreliable.
Downstream of the plate store and feeding apparatus conventional systems load the recording plate into the drum image scanner and in such equipment the plate may be supported on an internal or external surface of a drum for the recording of an image. Typically the loading of the plate into the image recording position is achieved using rollers or drive belts. As the exact positioning of the plate is particularly important, such systems include the use of guides and stops to ensure correct alignment with the image recording device. De-skewing of the plate is conventionally achieved when the plate is in the imaging position, by applying of a trail edge force via a sprung system which applies a variable force at different points depending on the skewing of the plate. However, this relies upon low frictional forces between the plate and the drum, particularly in the case of drums having a small radius of curvature.
It is also important to ensure that the surface of the plate conforms with that of the drum, i.e. each part of the plate surface is in contact with the surface of the drum. Prior art systems typically achieve this by either using mechanical means to apply a force to the plate causing it to bow and therefore conform with the drum, or by applying a vacuum to the drum surface causing the plate to be held in the conforming position. An example of the former method involves the loading of the plate into the apparatus such that the leading edge abuts some end stops. Subsequently, fingers are driven against the rear edge of the plate causing it to bow outwards and conform with the drum. As the apparatus may be used with different sizes of plates, the fingers in this case may be controlled with stepper motors or a sprung system to ensure the correct degree of movement based upon plate size information given to the apparatus controller by an operator.
In the second case where a vacuum system is used to conform the plate to the drum surface, it is conventional to provide a plurality of grooves within the surface of the drum. The grooves are connected to apertures and a vacuum is applied to the grooves using suitable means. The grooves are respectively attached to separate vacuum circuits which are either switched on or off using a plurality of electronically controlled valves in order to apply a vacuum to an area corresponding to the size of the recording plate.
In accordance with one aspect of the present invention we provide image recording apparatus for recording an image on a recording plate, the apparatus comprising;
a plate store for storing a stack of recording plates separated by interleaved sheets;
a drum image scanner on which recording plates are located for recordal of an image; and
a frictional feed apparatus for withdrawing recording plates from the store and feeding them singly to the drum image scanner.
In accordance with the second aspect of the present invention we provide a method of feeding recording plates from a plate store containing a stack of plates with interleaved sheets, to a drum image scanner, the method comprising withdrawing plates from the store and feeding them singly to the drum image scanner all under frictional control.
It has been found that frictional feeding apparatus can be used to successfully withdraw plates from a stack of interleaved plates and sheets in order to feed it to a drum image scanner. It has also been found that this feeding operation can be achieved without significant damage to the sensitive image recording surface.
In accordance with the third aspect of the present is invention we provide frictional feed apparatus for feeding single recording plates or single interleaved sheets from a stack of recording plates and interleaved sheets, to an output position, the apparatus comprising;
a nudger member for nudging a plate or sheet in the stack towards a feed location; and
a feed member and cooperating separation member at the feed location for feeding single plates or sheets to the output position;
wherein the contact surfaces of the nudger, feed and separation members are arranged so as to produce sufficient frictional force when in contact with either a plate or a sheet, such that plates or sheets are fed singly towards the output position.
The feed apparatus described is capable of withdrawing either recording plates or interleaved sheets in a similar manner and passing them to an output location. The apparatus may be used to withdraw plates or sheets from either the top or the bottom of a stack but preferably the topmost plates or sheets are withdrawn. The nudger member which preferably comprises a nudger roller engages the first plate or sheet in the stack and passes it to the feed location. Preferably the feed member is also a feed roller and the sheet or plate is passed into the nip of the feed member and the separating member. The nudger and feed members may be coupled by suitable means such as a belt, gears or a chain and sprocket. The separating member is preferably a retard pad which may present a surface inclined to the feed direction of the plate or sheet to ensure that the nip is maintained but to allow passage of the plates and sheets. The feed and separating members are preferably urged together using urging means. The separating member is preferably pivoted, with an angled pad underneath that is sprung to provide a nip force. Preferably the urging means comprises a spring attached to the retard pad. The spring therefore provides a force to oppose the movement of the separating member. It will be appreciated that the feed member may also be movable and provided with urging means.
In the event of a malfunction during a feed operation, it is desirable to include means to separate the feed and separating members. This is preferably achieved using a motor and eccentric cam.
It is desirable that only one member within the frictional feed apparatus drives the plate or sheet forward at any one time and therefore the apparatus preferably further comprises nudger member movement means to move the nudger member from the nudging position to a retracted position. This may be achieved using a rotatably mounted cam. In this case the nudger member may simply be returned to the nudging position under the action of gravity.
Preferably the apparatus further comprises stack position sensing means to detect when the nudger member is in the nudging position, in order to feed a plate or sheet from the stack. It will be appreciated that the position of the nudger member in contact with the stack need not be used to measure the position of the next item to be fed from the stack as the position of this item could be measured directly using appropriate sensing means, for example using an optical sensor.
Advantageously the use of the feeding apparatus according to the present invention greatly improves the speed with which plates can be drawn from the plate store, for example a feeding speed of 80 mm per second can be achieved.
The action of shearing a recording plate from the stack requires a smaller force than that required to lift the plate vertically and it will be appreciated that if a vacuum exists between the plate and interleaved sheets, it is easier to separate these by shearing rather than separating them in a direction normal to their contact surface. Typically, the force required to separate a plate by shearing is in the range 1N to 20N, whilst the force required to separate a plate by lifting ranges between 2N and 70N. The magnitude of these forces is affected by the size of the plates, the environmental conditions, the condition of the plate stack and storage conditions.
For example plates stored horizontally at the bottom of a stack of boxes prove very difficult to separate. The use of a smaller force is advantageous as it reduces the risk of damage to the surface of the recording plates.
It is also advantageous to use similar apparatus to remove both recording plates and interleaved sheets from the stack. Preferably these then require selection at a position downstream of the feed location. Typically the selection is achieved by placing a diverter in the path of the plate and sheets in order to divert only recording plates in a first direction towards the drum image scanner but to divert interleaved sheets in a second direction. Preferably this is achieved using sensing means, provided downstream of the feed location, in order to determine the presence of a plate or a sheet. Preferably the sensing means comprises an inductive sensor in order to detect the presence of a recording plate. Similarly an alternative or additional sensing means may be provided comprising a first radiation source to illuminate the upper or lower surface of the plate or sheet and a first sensor to detect any reflected radiation at a predetermined location. The material and surface properties of recording plates and interleaved sheets cause radiation to be reflected in differing ways which allows them to be distinguished. As a variety of mis-feed scenarios can be envisaged involving a combination of plates or sheets, preferably this sensing means further comprises a second radiation source and a second sensor positioned such that the sensing means monitors the reflected radiation from both the upper and lower surfaces of the plate or sheet. The sensing means may then distinguish for example between a singularly fed plate and a plate and sheet fed in combination with the plate topmost of the two.
Preferably the diverter is arranged to be responsive to the output of one or more of the sensing means in order to prevent an interleaved sheet from being mistakenly fed to the drum image scanner.
As the nudger and feed members contact the sensitive surface of the recording plate, it is preferable that they are kept free of debris and therefore the apparatus preferably further comprises vacuum cleaning means to clean the nudger member and/or the feed member. The vacuum cleaning means may also include the use of a brush to dislodge debris from the nudger or feed members. Although the vacuum cleaning means may be attached to the support member of the nudger or feed members, preferably the vacuum is applied through ducting provided within the support member.
In accordance with a fourth aspect of the present invention we provide a method of withdrawing recording plates or interleaved sheets from a stack of recording plates and interleaved sheets, the method comprising nudging a plate or sheet in the stack towards a feed location using a nudger member and feeding the plate or sheet to an output position using a feed member with a cooperating separating member at the feed location wherein the contact surfaces of the nudger, feed and separation members are arranged so as to produce sufficient frictional force when in contact with either a plate or a sheet, such that plates or sheets are fed singly towards an output position.
Typically the method further comprises moving the nudger member away from the nudging position when the first plate or sheet is in contact with the feed or separating members. Preferably the method further comprises the diverting of the interleaved sheets away from the output position.
Once the recording plate has been fed to the output position, it may then be engaged by an input module. An example of such an input module comprises a friction belt which transports the plate over one or more rotatably mounted rollers. Preferably, the belt and rollers are driven at substantially the same velocity. Advantageously the belt and rollers are arranged such that only the belt contacts the sensitive surfaces of the recording plates whereas the opposite surfaces are contacted with the rollers. Such a cooperating roller and belt system transports the plate into the drum of the image scanner.
Once within the drum, the plate is then engaged with plate alignment apparatus which, in accordance with a fifth aspect of the present invention, is provided for the alignment of a recording plate on a drum of a drum image scanner, the apparatus comprising; an elongate pusher bar extending along the drum and substantially parallel to the cylindrical axis of the drum, wherein the bar has a pusher surface having an elongate axis extending along the bar and substantially parallel with the cylindrical axis, and means for moving the bar in a circumferential manner following the surface of the drum, the elongate axis of the pushing surface remaining substantially parallel to the cylindrical axis, wherein in use, the pusher surface engages with the rearmost edge of a recording plate and urges it to a first position for the recordal of an image.
Typically the partial loading of the plate into the drum image scanner involves passing the plate over the elongate pusher bar of the plate alignment apparatus. The bar may be fitted with additional rollers which effectively extend the path of the input module along which the plate is fed. Once the rearmost edge of the plate has passed the pusher bar preferably the pusher bar drops and engages the rearmost edge of the plate. It is preferable that the pusher bar is urged against the surface of the drum to ensure that it correctly engages with the plate""s rear edge.
The movement of the bar may be achieved using pinions mounted to the pusher bar and corresponding tracks mounted upon the drum, the bar being fixed to the drum using suitable bearings mounted on a rim provided at the drum edges.
It is envisaged that the alignment means according to the present invention may be used together with vacuum conforming means to conform the plate. Alternatively it may be used to apply the required bending force to the plate as part of a mechanical conforming means.
The circumferential motion of the pusher bar drives the plate from the rear towards a punching means which preferably comprises retractable end stop pins against which the plate is urged. The retractable end stops are positioned within the drum to abut the leading edge of the recording plate when the plate is located in the first position. Typically at least one sensor is positioned to detect the leading edge of the plate when it approaches the pins.
Preferably lateral correction means are also fitted to the punching means to correct the position of the plate in a direction substantially parallel to the cylindrical axes of the drum. Such means may comprise pins which are symmetrically opposed about the centre of the drum""s cylindrical axis. The pins in this case therefore remain equidistant from the centre of the drum and are moved inwards in order to engage with the sides of the plate causing it to be positioned centrally.
Preferably the apparatus further comprises a slip clutch mounted to the moving means of the pusher bar. This allows the bar to be pushed against the rearmost edge of the plate after lateral position correction, to effect the final plate positioning in the apparatus, the force with which the plate is urged against the end stop pins being determined by the slip clutch.
Advantageously the punching means is arranged to remove at least one portion of the recording plate. This may be achieved preferably using at least one punch indenter which passes through the plate during punching and holds the plate in position during later imaging.
Having positioned the plate within the apparatus it is advantageous to conform the plate to the surface of the drum using vacuum means.
In accordance with a sixth aspect of the present invention we provide a drum of a drum image scanner for locating recording plates of differing dimensions on the surface of the drum, the surface of the drum on which the plates are located containing a plurality of circumferentially extending axially spaced grooves, wherein each groove contains at least one aperture, all the apertures being connected to a vacuum source whereby all the apertures receive substantially the same vacuum level at all times, the apertures being located such that, when at least part of the length of a groove is covered by a plate, the covered portion contains the aperture or apertures opening into that groove.
This arrangement is advantageous in that complex and expensive valve mechanisms are not required for the performance of the invention. Typically each groove is closed at one end such that it does not extend to the edge of the drum, and contains only a single aperture. Advantageously this arrangement of grooves and apertures in connection with a vacuum source provides the capability of conforming various sizes of recording plates to the surface of the drum without the need for complicated vacuum circuits.